How to rotate multiple objects towards one point?
Perhaps DupliVerts are too inflexible for what you need, but their ability to orient objects according to the parent object's vertex normals certainly makes it easy to have duplicates point toward the center, as illustrated here:
 

To make the above, simply parent the master cone (in center) to the spiral, then go to the spiral's Properties->Object tab, where you will find a Duplication category (in red box above). Select Verts, then check Rotation (in green box above). Then simply rotate the master cone to re-orient all the other cones however you want. Be aware that whether-or-not the master cone has its rotation applied (Ctrl-a) makes a difference; try both ways to see which suits in this instance.

Also know that for finer detail and ability to scale seeds differently, you could do a Make Duplicates Real for all the DupliVerts, then give each a Track To constraint oriented toward an empty at the center.

By the way, the curve was created via Add Curve->Extra Objects, then converted to mesh for this use. You might need to turn Extra Objects on in User Prefs.